In the prior art, it has long been known to power a gas discharge lamp such as a neon lamp or fluorescent tube from an A.C. source by means of a conventional voltage step-up transformer or ballast. These devices suffer from a number of drawbacks. To power lamps of significant size, bulky and heavy copper windings formed on massive magnetic cores, usually of a laminated silicon steel are usually required. In addition to being relatively expensive, such construction may require the use of thermal protection to safeguard against overheating. Increased weight and bulk impose difficulty in mounting the driver circuitry in an aesthetically pleasing manner in lighted signs, displays, fixtures or the like. Units operating from ordinary line voltage are also often subject to a tendency to generate significant radio frequency interference as well as an annoying audible hum and are not readily dimmable.
In an attempt to overcome the aforementioned limitations, there have arisen in the prior art a number of gas discharge lamp drivers incorporating smaller and less massive transformers switched by semiconductor devices at high, inaudible frequencies. Such drivers may be powered either directly from a D.C source or from an A.C. source by way of A.C. to D.C. converter circuitry. For example, U.S. Pat. No. 3,467,887 to Skirvin shows both A.C. and D.C. powered circuits in which a transformer, whose secondary is connected to a gas discharge lamp, includes at least two primary windings. The primary windings are coupled to a switching transistor and diode to form a regenerative oscillator operating at a frequency above the audible range.
U.S. Pat. No. 2,982,881 to Reich shows lamp power supplies of the regenerative, single transistor type as well as types wherein the transformer primary is connected to a pair of transistors in a push-pull arrangement. However, units of the latter type also generate excessive radio frequency interference and are subject to reduced light output in cold operating environments. The latter problem has also been known to exist in the step-up transformer type drivers discussed above.
In each of the aforementioned circuits, the maximum current which can be driven through the primary winding is limited by the rating of a single transistor. In order to adapt these circuits to handle more power, a transistor of a higher power rating would be required. Unfortunately, the switching speed of bipolar transistors ordinarily decreases as the power rating of the transistor decreases. The cost of bipolar transistors also generally increases disproportionately with increases in power rating. Because most of the power dissipation of a transistor operating in a blocking oscillator takes place as the transistor changes its conductive state from conducting to non-conducting, higher power circuits have heretofore been less efficient as well as disproportionately more expensive than their lower power counterparts.
German Offlenlegungsschrift No. 26 10 944 discloses a blocking oscillator fluorescent lamp ballast. The ballast includes a pair of primary windings coupled to one another by way of a pair of bipolar transistors, the latter of which are connected in parallel with one another to improve efficiency by reducing the internal resistance of the oscillator. However, unless more expensive matched transistors are used, the base circuit of at least one of the transistors must be connected to a resistor of low ohmic value selected to equalize the load on both transistors. The need to trim a resistance value to match the load on the transistors is an impediment to low cost manufacturing of the circuit.